Sarah Shadlock makes her first appearance in The Silkworm, at Matthew’s mother’s funeral. She is a friend of Matthew’s from their university days, and she is first seen embracing Matthew in a way that Robin thinks is “a little more lascivious, perhaps, than was appropriate in the circumstances.”
In Career of Evil, after socializing with Sarah and her boyfriend, Tom, it is observed that Robin and Matthew always seem to quarrel. Robin regards Sarah as a “shit stirrer,” who at university “had tried as hard as she could to win Matthew away” from her.
Physically, Sarah is described as looking like Strike’s girlfriend Elin, with Nordic-blonde beauty (but Sarah’s not as beautiful as Elin, in Robin’s estimation).
In the TV series adaptation, Sarah is portrayed by actress Antonia Kinlay.